The invention relates to a transmitter of radio signals, referred to as output modulated signals, which are modulated at least in amplitude according to a discrete plurality of predetermined separate amplitude levels on the basis of digital control signals, comprising,
modulation means adapted to convert the digital control signals into modulated signals, referred to as input modulated signals,
solid-state power amplification means including at least one power transistor, at least one of which is an output power transistor delivering output modulated signals,
bias means including at least one voltage source, and adapted to bias each power transistor of the amplification means.
Transmitters of modulated radio signals have been known for a long time and can be extremely varied. The modulation may be phase and/or amplitude and/or frequency modulation. In the case of amplitude modulation, and also in the case of certain phase modulations, the modulated signals received at the input and those delivered at the output of the power amplification means have an amplitude which varies permanently in a discrete fashion, that is to say being able to take one of the values of a predetermined discrete plurality of separate amplitude levels.
For this reason, it is necessary to over-engineer the amplification means and to bias the power transistors with a large input offset in order to avoid saturation and make it possible to deliver the various output powers with good linearity. However, it is found that the efficiency of a power transistor varies, for a given bias, as the amplitude of the signals at the input and at the output of the transistor. Therefore, for the lowest signal amplitude levels, the efficiency of a transistor drops considerably. Moreover, such a drop in efficiency is highly detrimental in all applications in which the electrical energy source for supplying the transmitter is not inexhaustible, and is formed in particular by accumulators or cells, and more generally when this electrical energy is expensive.
In these applications, the use of modulation such as M-QAM amplitude modulation with M states has hitherto been discarded, in spite of the significant advantages which such modulation can provide (in particular a small spectral size for the same bit rate). Such is, in particular, the case on board space systems such as satellites, and more particularly micro-satellites, in which energy and cost economies as well as the problem of simplicity and speed of manufacture are crucial. Such is also the case with portable radio communication equipment.
This problem is generally encountered for each of the power transistors of the amplification means, but more particularly at least for the output power transistor(s), whose operation largely determines the overall electrical consumption of the transmitter. The reason is that, in the case of several amplification stages, the electrical consumption is known to be the greatest in the output stage delivering the output signals.
In order to overcome this problem, a technique referred to as envelope elimination/restoration EER has been proposed, in which the envelope of the modulated signals coming from the modulator is detected, the envelope of the modulated signals is eliminated with the aid of an amplitude limiter, the output power transistor is saturated with a high input power, a high-frequency PWM (“pulse width modulation”) switching converter, which is driven according to the envelope detected previously, is used for the bias (the switching frequency needing to be much higher than that of the envelope of the modulated signals), so that the variation in the bias provided by this converter makes it possible to restore the envelope on the modulated signals at the output of the output power transistor. This technique is, however, limited by the switching-frequency values and is not therefore easy to apply at very high envelope frequencies, especially in the field of high-speed digital data transmissions (typically several megabits per second or tens of megabits per second between space systems—in particular satellites—and the earth). Furthermore, production of the envelope detector, the limiter and the converter is relatively complex—especially in the field of high-speed transmissions in which the modulated signals have a microwave carrier.
It should be noted, in particular, that when the complex envelope of a microwave signal passes through the value 0, the envelope of the real signal has a stationary point which, with this EER technique, induces an abrupt voltage variation with spectral components whose frequencies are extremely high. Furthermore, this EER technique demands excellent overall linearity of the system formed by the amplification means and the bias converter, throughout the dynamic range of the envelope of the signal.
Similarly, the document “MICROWAVE POWER AMPLIFIER EFFICIENCY IMPROVEMENT WITH A 10 MHz HBT DC-DC CONVERTER” Gary Hanington et al, International Microwave symposium, Baltimore, 1998 IEEE MTT-S Digest WE2C-6 pp. 589–592, teaches detecting the power of the input modulated signal of the amplifier with an RF coupler and an envelope detector, and driving the value of the bias voltage of the power amplification stage provided by a PWM switching converter. There again, this solution is frequency-limited (the modulated signals being able to have a variation spectrum not exceeding 2 MHz for a switching frequency between 10 and 20 MHz). It is furthermore relatively complex to produce, in particular when the modulated signals have a microwave carrier. This is because the circuits through which microwave signals travel are known to be complex, expensive and difficult to tune. It should furthermore be noted that any error or any noise incorporated in the input modulated signals is echoed in the converter and the bias of the power transistor. This solution therefore tends to amplify the errors and noises and has imprecise operation.